Just like your mom kept telling you after you finished with a craft project as a child, clean up after yourself. Same thing here, it's just digital clean up. Take a few minutes after your finished with your illustration.
Remove unused swatches, don't use spot colors, delete stray points, close open paths, and expand effects such as strokes and brushes.
If you use Adobe Illustrator, here is a great script written by Michael Schmeling. This script allows you to find all those stay points, open paths, and other unwanted elements. It's a must have for stock artists.
Download it from his webpage. Thank you Michael Schmeling.
Curves and Paths
Be diligent, zoom in, hit "command (pc ctrl) + y" if you need to, and look at your paths. Make sure they are clean and smooth. Or if it's stylized, be consistent. We don't want sloppy work. Also fully render shapes when you have two objects overlap. This gives the buyer the ability to rearrange your art making it more flexible.